(1). Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, printer, and facsimile, which can form a full-color image, and particularly relates to a full-color image forming apparatus which forms an image on a transfer material such as a recording sheet using image holding components.
(2). Description of the Related Art
Tandem type full-color image forming apparatuses, as examples of a conventional full-color image forming apparatus which can form a full-color image, have come to dominate in this field in recent years. The tandem type full-color image forming apparatus has four photosensitive drums. Axes of the photosensitive drums are parallel to one another and orthogonal to the direction of the length of a transport belt. Toner images corresponding to reproduction colors cyan, magenta, yellow, and black (respectively referred to as C, M, Y and K hereinafter) are separately formed on the photosensitive drums, and are sequentially transferred onto a transfer material which is transported on the transport belt. The toner images are superimposed on the transfer material to form a full-color image.
When forming a black image using this tandem type full-color image forming apparatus (hereinafter, the black image is referred to as the "monochrome image" and a mode for forming a monochrome image is referred to as the "monochrome mode"), the user sets only monochrome documents to the apparatus and specifies the monochrome mode using an operation panel or the like, so that image formation is performed using only a photosensitive drum used for the black image formation. In this case, the four photosensitive drums are all driven. However, the photosensitive drums for C, M, and Y are not used for the black image formation although being rotated. By doing so, the construction used for the monochrome image formation can be minimized, thereby simplifying the construction of an image forming mechanism. Some types of conventional tandem type full-color image forming apparatuses employ this technique (referred to as the "first technique" hereinafter).
In contrast to the first technique, there has been another technique (referred to as the "second technique" hereinafter) where the rotations of the photosensitive drums corresponding to the reproduction colors which are not used for the monochrome image formation are stopped and these photosensitive drums are separated from a surface of the transport belt in the monochrome mode. This second technique is disclosed in Japanese Laid-Open Patent Applications No. 3-288173, No. 6-258194, and No. 6-102776.
Using the first technique, all the photosensitive drums are driven even in the monochrome mode, so that cleaning components come in contact with the corresponding surfaces of the photosensitive drums. Here, toner is not attracted to the photosensitive drums for C, M, and Y, and accordingly, excess frictional resistance is produced on the surfaces of the photosensitive drums that are not used for the monochrome image formation. This shortens the lifespans of the photosensitive drums, and leads to wear and tear on the cleaning components.
Using the second technique, a switching operation is performed to switch states between where the transport belt is separated from the photosensitive drums (referred to as the "separated state" hereinafter) and where the transport belt is pressed against the photosensitive drums (referred to as the "pressing state" hereinafter). When the switching operation is performed, operations including the image forming operation, such as the exposure on the photosensitive drums and the toner image transfer onto the transfer material, and the paper feeding operation need to be temporarily stopped. Due to this suspension, the image forming efficiency is not expected to be high.
In recent years, while images to be formed have been increasingly multicolored, there are still monochrome documents, such as documents including only characters. A copying operation may be requested for a mixture of color and monochrome documents.
To automatically perform the copying operation for the mixture of color and monochrome documents, each document needs to be judged whether it is color or monochrome before the image formation is performed. However, it is impossible to perform this automatic copying operation using the first and second techniques since the stated conventional image forming apparatus has no judging functions to judge whether a document is color or monochrome.
Suppose that the judging function for judging whether a document is color or monochrome is added to the conventional image forming apparatuses so that the mode can be switched between the color mode and the monochrome mode in accordance with the judgement result. Here, the conventional image forming apparatus using the first technique still has the stated problems, such as the shorter lifespans of the photosensitive drums and the wear and tear on the cleaning components, since the monochrome image formation is performed with all of the photosensitive drums being driven and with the three photosensitive drums being rotated without taking part in the monochrome image formation.
Also, the conventional image forming apparatus using the second technique, where the rotations of the photosensitive drums not used for the monochrome image formation are stopped and the transport belt is separated from those photosensitive drums in the monochrome mode, still has the stated problem that the image forming efficiency is not so high. In addition to this problem, the transport belt is repeatedly separated from or pressed against the photosensitive drums in accordance with the judgement results during a series of copying operations. As a result, image deterioration may occur on a reproduced image due to the mechanical vibrations caused by the switching operation.